Graded junctions are used in solid state transistors to enhance device performance. The application of a graded junction in the base region of a bipolar transistor results in the formation of a nonuniform energy gap. A graded bandgap can be employed to reduce base transit time and thus increase the device speed, see H. Kroemer's "Two Integral Relations Pertaining to the Electron Transport through a Bipolar Transistor with a Nonuniform Energy Gap in the Base Region", Solid-State Electron, Vol. 28, (November 1985), pp. 1101-1103. The bandgap of silicon can be varied by the introduction of dopants, the formation of alloys, e.g. silicon/germanium (SiGe), and/or the introduction of strain into the crystal lattice. Combinations of all three of these phenomena have been used to produce very high speed graded SiGe-base heterojunction bipolar transistors, such as reported in G. L. Patton et al's. "75-Ghz f.sub.t SiGe-Base Heterojunction Bipolar Transistors," IEEE Electron Device Lett., Vol. 11 (April 1990), pp. 171-173, and "Graded-SiGe-base, Poly-emitter Heterojunction Bipolar Transistors, " IEEE Electron Device Lett., Vol. 10 (December 1989), pp. 534-536). In addition, in accordance with J. D. Cressler et al's., "Sub-30-ps ECL Circuit Operation at Liquid Nitrogen Temperature Using Self-aligned Epitaxial SiGe-base Bipolar Transistors," IEEE Electron Device Lett., Vol. 12 (April 1991), pp. 166-168, these devices have many advantages over conventional silicon devices for high speed digital and microwave devices by providing higher emitter injection efficiency, lower base resistance, lower base transit times, and superior low temperature speed and gain.
The SiGe alloys employed in the devices are epitaxially grown onto single crystal (100) silicon and have crystal lattices which assume the smaller Si lattice spacing. The resulting strain makes an important contribution to the bandgap reduction in the commensurately grown alloy material. As discussed above, this material is used as the base region of these transistors. The alloys must be formed at temperatures below those conventionally used for epitaxial growth (above 900.degree. C.) and subsequent processing must also remain at temperatures below about 850.degree. C. to avoid strain relaxation through the formation of misfit dislocations at the alloy/silicon interface.
Limited reaction processing (LRP), ultrahigh-vacuum/chemical vapor deposition (UHV/CVD), and molecular beam epitaxy (MBE) have been used to produce strained heterojunction bipolar transistors. Further insight into these techniques, in addition to those referred to above is disclosed in C. A. King et al's., "Si/SiGe Heterojunction Bipolar Transistors Produced by Limited Reaction Processing," IEEE Electron Device Lett., Vol. 10 (February 1989); G. L. Patton et al's., "Silicon-Germanium-base Heterojunction Bipolar Transistors by Molecular Beam Epitaxy," IEEE Electron Device Lett., Vol. 9 (April 1988), pp. 165-167; and T. Tatsumi et al's "Si/Ge.sub.0.3 Si.sub.0.7 /Si Heterojunction Bipolar Transistor Made with Si Molecular Beam Epitaxy," Appl. Phys. Lett., Vol. 52 (March 1988), pp. 895-896.
Thus, a continuing need exists for a method of fabricating a graded junction device relying on a pulsed laser rapidly melting of the layers of an amorphous or polycrystalline film of doped silicon over an amorphous or polycrystalline film of germanium and on a subsequent epitaxial recrystallizing to form an epitaxial silicon-germanium alloy with a graded heterojunction.